In recent years, contactless communication technologies have been established for exchanging signals through electromagnetic induction and are expanding their applications in transportation tickets and electronic money. Such a contactless communication function is more and more applied to mobile phones and is promising for future development. Not to mention Near Field Communication (NFC) through electromagnetic induction, IC tags with reading/writing distances of several meters have been also introduced into the market of logistics. Since allowing not only contactless communication but also simultaneous power transfer, the contactless communication technology may be used even in an IC card that itself does not have a power source such as a battery.
Besides, due to the specification, such as Qi, for contactless charging of a portable terminal apparatus, the power transfer technology of establishing inductive coupling or magnetic resonance between a charging device (a power transmitter) and a power-receiving device (a power receiver) through antenna coils included in these devices is gradually becoming popular.
In a system using such contactless communication (or contactless charging), resonant capacitors are connected to the loop antennas for communication and power transfer between a reader/writer (a charging device) and a contactless data carrier (or a power-receiving device). Furthermore, tuning a resonant frequency, which is determined by constants L and C of the loop antennas and the resonant capacitors, to a prescribed frequency of the system allows stable communication and power transfer, as well as maximization of a distance, between the reader/writer and the contactless data carrier.
However, the constants L and C of the loop antennas and the resonant capacitors are subject to several variable factors and are not always predictable. For example, a contactless data carrier or a power-receiving device may include a loop antenna made from copper foil patterns as a cost-reduction measure, and the L value varies due to a change in pattern width. As a cost-reduction measure, a resonant capacitor may also have copper foil of an antenna substrate configured to serve as electrodes and a resin of the substrate configured to serve as a dielectric, and the capacitance value changes depending on the width, the length, and interval of the copper foil piece. In the case of an IC card, since upper and lower sides of the IC card are ultimately laminated by protective films, the capacitor's capacitance is subject to an even greater variation due to the influence of the protective films. This raises the need for additional man-hours for, for example, regulating the capacitance value of the resonant capacitor by regulating an electrode area by trimming the copper foil pattern, as anticipative regulation in expectation of a frequency shift following the lamination.
When the resonant frequency is shifted due to the aforementioned various factors, the communication status might be unstabilized, and the communication distance might be reduced. One proposed method to address the problems is to regulate, in an antenna module including an antenna coil through which a magnetic flux from a reader/writer passes and a resonant circuit that efficiently converts a change in the magnetic flux into voltage, the resonant frequency by regulating the capacitance of a resonant capacitor in order to stabilize communication.
A contactless data carrier and a contactless chargeable power-receiving device, including an IC card, are often used in an apparatus targeted at portability as described above, and therefore, more compact and thinner resonant capacitors and loop antennas are strongly demanded. Furthermore, the trend of modulization of readers/writers and charging devices will even increase the need for miniaturization and thinning.
Incidentally, a ferroelectric thin film capacitor using a ferroelectric thin film of barium titanate or the like has a high dielectric constant per unit area and is suited for miniaturization and thinning, and accordingly, is being considered as a candidate for the above applications. However, the ferroelectric thin film capacitor still suffers from the problem of a large initial variation in the capacitance value and a large dependency of the capacitance value on temperature. The ferroelectric thin film capacitor is expected to find a wider range of applications if the above problem is attempted to be corrected also by the apparatus.